01 - Combine the drained tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, salt, and black pepper in a mixing bowl. Mix thoroughly until all ingredients are evenly distributed and the mixture is creamy and well combined.
02 - Place the tortillas on a clean work surface. Divide the tuna mixture evenly, spreading half onto the center of each tortilla. Sprinkle half the cheddar cheese over each portion. Add tomato slices if desired.
03 - Fold both sides of each tortilla inward toward the center. Starting from the edge closest to you, roll up the tortilla tightly to enclose the filling completely.
04 - Melt the butter in a large skillet over medium heat. Place the wraps seam-side down in the pan. Cook for 2 to 3 minutes per side, pressing gently with a spatula, until golden brown and crispy with melted cheese inside.
05 - Transfer the wraps to a cutting board. Slice each wrap diagonally in half and serve immediately while warm and crispy.